Medical injection-molded products, due to their direct contact with the human body or use in precision medical scenarios, impose stringent requirements on surface quality. Surface defects such as flow marks, silver streaks, gas marks, weld lines, and sink marks not only compromise appearance but also risk bacterial growth or structural weakness, directly threatening patient safety. This article systematically addresses solutions to surface quality issues in medical injection molding from five dimensions: material selection, mold design, process control, equipment maintenance, and quality inspection.

1. Material Selection: Quality Control from the Source

Medical injection molding materials must meet biocompatibility, chemical resistance, and transparency requirements while balancing processability.

• Material Compatibility: Avoid materials containing plasticizers or low-molecular-weight additives, which may exude and cause surface whitening or hazing. For example, medical-grade PC (polycarbonate) should use pure resin without recycled materials.
• Flow Optimization: For thin-walled or complex products, high-flow materials (e.g., PPSU, PEEK) reduce filling deficiencies or weld lines. However, balance flow with rigidity to prevent surface scratches.
• Drying Treatment: Hydroscopic materials (e.g., PA, PBT) must be dried to ≤0.02% moisture content to prevent hydrolysis-induced silver streaks or bubbles. Use dehumidifying dryers with temperatures following supplier recommendations.

2. Mold Design: Details Determine Success

Mold design critically impacts surface quality, requiring optimization of runner systems, cooling systems, and venting structures.

• Runner Balance: Hot runner systems minimize cold slugs, reducing flow marks. For multi-cavity molds, CAE simulations optimize runner dimensions for uniform filling.
• Cooling Efficiency: Evenly distributed cooling channels prevent localized shrinkage. For transparent parts, conformal cooling (3D-printed waterways) improves efficiency by 30%+.
• Venting Optimization: 0.02–0.05mm vents at parting lines, ejector pins, and slides prevent gas compression burns or gas marks. Vacuum venting is recommended for high-gloss products.

3. Process Control: Precision Parameterization

Injection molding parameters must dynamically adjust based on material properties and product structure, focusing on temperature, pressure, speed, and time.

• Temperature Control:
  • Barrel Temperature: Exceed material melting point by 10–30°C but avoid overheating. For PPSU, 320–350°C is ideal.
  • Mold Temperature: High-gloss products require 80–120°C to minimize weld lines; thin-walled parts use 40–60°C for shorter cooling.
• Injection Speed: Multi-stage speed control prevents cold slugs (initial high speed) and reduces internal stress (final low speed). For transparent parts, speed gradients ≤20mm/s are advised.
• Packing Pressure: Sufficient to compensate for shrinkage but avoid flash or residual stress. Typically 60–80% of injection pressure.
• Back Pressure: Too low causes uneven plasticization; too high increases shear heat. Medical-grade materials usually require 5–15 bar.

4. Equipment Maintenance: Preventive Management

Equipment condition directly affects process stability, necessitating regular maintenance.

• Screw Cleaning: Clean with purging compounds every 24 hours or during material changes to prevent carbon deposits causing black spots.
• Hot Runner Maintenance: Check temperature controller accuracy (±2°C tolerance); recalibrate if exceeded to prevent material degradation.
• Mold Care: Polish every 5,000 cycles to remove surface scratches; replace ejector pin grease every 20,000 cycles to prevent sticking and part whitening.

5. Quality Inspection: Full-Process Traceability

Medical injection molding requires end-to-end traceability, from raw materials to finished products.

• Inline Inspection: Use vision systems (e.g., CCD cameras) with AI algorithms for real-time defect detection and sorting.
• Sampling Inspection: Follow AQL standards, focusing on critical metrics like weld line depth (≤0.05mm) and shrinkage rate (≤0.5%).
• Reliability Testing: Simulate real-world conditions for chemical resistance and fatigue testing to ensure long-term surface stability.

Conclusion

Resolving surface quality issues in medical injection molding requires a lifecycle approach, targeting zero defects through material selection, mold design, process optimization, and equipment maintenance. Digital technologies like CAE simulations and AI-driven inspection, combined with lean manufacturing, systematically reduce defects to meet medical industry standards. Future advancements in biobased materials and nano-coatings promise higher-dimensional breakthroughs in surface quality.